Analog optical fiber communication system, and laser adapted for use in such a system

ABSTRACT

Lasers for use in multichannel analog optical fiber communication systems (e.g., of the type contemplated for CATV) have to meet very stringent requirements, including high linearity. DFB lasers are advantageously used in such communication systems. Typically only a relatively small percentage of the nominally identical DFB lasers on a wafer meet the specifications. It has now been discovered that the likelihood that a given DFB laser will meet the requirements is substantially increased if the laser comprises means that are adapted for producing a non-uniform photon density in the laser cavity, with the density of photons being larger in the rear portion of the cavity than in the front portion, such that during operation of the laser the gain in the back portion is substantially independent of the laser current, whereas the gain in the front portion is a function of the laser current. Exemplarily, lasers according to the invention have power asymmetry less than about 5. Preferred embodiments are AR/HR lasers with KL in the range 1.6-2.5. Optionally lasers according to the invention have a phase shift located in the back portion of the grating, a split contact, and/or a non-constant mesa width.

FIELD OF THE INVENTION

This invention pertains to optical multichannel analog communicationssystems, and to semiconductor lasers adapted for use in such systems.

BACKGROUND OF THE INVENTION

The concept of transmitting several television channels through opticalfiber using analog intensity modulation of the output of a semiconductorlaser diode has recently been receiving considerable attention. Asproposed in the prior art, this would involve transmission ofmulti-channel amplitude modulated-vestigial side band (AM-VSB) signals,as used in present day cable television (CATV) systems, in an opticalfiber transmission medium. Such an arrangement would be useful in a CATVtrunk system or in a fiber-to-the-home network. Optical fibertransmission systems that use frequency division multiplexing overcomecompatibility problems and have advantages such as simplicty of design,reduced bandwidth requirements for lightwave components, and much lowercost, as compared with optical time division multiplex (TDM) systems.

The wide bandwidths of semiconductors laser diodes and optical fibersmake analog sub-carrier modulation an attractive technology. Severalsignals at different sub-carrier frequencies, each signal representingone of the television channels to be multiplexed, are summed and appliedconcurrently to the input of the laser device. The input informationsignal is a set of frequency-modulated sub-carriers at differentfrequencies, e.g., frequencies ω₀, ω₁, ω₂, . . . . The resulting laserinjection current comprises a dc bias level plus the set offrequency-modulated sub-carrier signals. The magnitude of the opticaloutput power from the laser fluctuates with the magnitude of the laserinjection current. The resulting sub-carrier frequency divisionmultiplexed (FDM) optical output signal is applied to an optical fiberfor transmission over an extended distance. After transmission throughthe fiber the optical signal is detected by appropriate means, e.g., aPIN diode, and the resulting electrical signal is processed byconventional means to recover the individual signals. See, for instance,R. Olshansky et al., Electronics Letters, Vol. 23(22), pp. 1196-1197(1987).

Multi-channel amplitude modulated signal transmission requires speciallimitations on the power, the non-linearity, and the intensity noise ofthe transmitting laser diode. For adequate system performance, laseroutput light intensity must be a very nearly linear function of thelaser drive current under large-signal modulation. Strict limitations onlaser nonlinearity are required because of the wide dynamic range of theNational Television Systems Committee (NTSC) standard video format.Exemplarily, in the NTSC standard video format, the ratio of themagnitude of the carrier to the magnitude of the total third orderintermodulation distortion products at the carrier frequency must beless than -60 dBc. Similarly, the peak second-order distortion, i.e.,the sum of several tens of two-tone products (or the ratio of thecarrier to the largest composite second-order peak), must be less than-60 dBc. To obtain such high signal quality in view of the large numberof distortion products, the transmistting laser light-versus-currentcharacteristic must be extremely linear.

In a system that uses frequency division multiplexing any nonlinearityin the laser diode characteristic will result in intermodulation noise.Laser nonlinearities create energy transfers from the appliedsub-carrier frequencies to, among others, those frequencies that are thesum and difference frequencies of all of the pairs of applied signalfrequencies. Such energy transfer cause undesirable intermodulationdistortion and interference, both of which can limit the performance ofthe transmission system.

There are several known causes of nonlinearity in semiconductor laserdiodes. Some of the causes of nonlinearity are high frequency relaxationoscillations, low frequency heating effects, damping mechanisms, opticalmodulation depth, leakage current, gain compression, and nonlinearabsorption. The resulting effect of the distortion and interference is adegradation in the signal-to-noise ratio for the signal, as receivedfurther along in the system.

An experimental sub-carrier frequency division multiplexed, opticalcommunication system having sixty frequency-modulated channels in the 2GHz to 8 GHz band has been operated with a 56 dB weightedsignal-to-noise ratio. Other arrangements using microwave carriers forsubscriber loop transmission have put (1) five frequency-modulated videochannels is the 150 MHz to 300 MHz band and (2) ten frequency-modulatedvideo channels in a C-band satellite signal in the 4.9 GHz to 5.2 GHzband.

The currently most attractive scheme for multiplexing multiple videochannels onto a continuous-wave laser output involves amplitudemolulated-vestigial sideband signal multiplexing. Some previouslyavailable semiconductor lasers can exhibit distortions approaching therequired low levels. However, typically only a small fraction of a givenbath of otherwise suitable lasers meet the distortion requirements,requiring extensive noise measurements to identify those lasers thathave sufficiently low distortion. U.S. patent application Ser. No.420,867, incorporated herein by reference, discloses a method ofproducing lasers that includes a simple technique for identifying lasersthat will have low distortion and thus may be suitable for use in amultichannel analog fiber communication system. Such a system isdisclosed in U.S. patent application Ser. No. 420,849, also incorporatedherein by reference.

However, even though there now exits a method that permits easyidentification of lasers that have low distortion, the number of laserson a given chip that have requisite low distortion typically is quitelow. Such low yield of course is highly undesirable since it results inrelatively high unit cost of acceptable lasers. A laser design than canresult in increased yield of low distortion lasers thus would havesubstantial economic significance. This application discloses adistributed feedback (DFB) laser having novel design features that canresult in increased yield of lasers acceptable for use in multichannelanalog communication systems.

Various aspects of DFB laser design have previously been considered witha view towards optimizing performance of such lasers in digitalapplications, including coherent optical fiber transmission systems.This work generally aimed at, inter alia, relatively high slopeefficiency ("slope efficiency" is defined herein as the maximum value ofdL/dI of the laser, where L is the radiation output power at the frontfacet of the laser, and I is the laser drive current) and, consequently,relatively high output power asymmetry between the front and back facetsof the lasers. Other important design criteria for digital laserapplications include spectral purity (or "side mode suppression ratio"),chirp, and linewidth. To optimize the performance of devices in theserespects the use of various facet coatings and phase shift have beenconsidered.

For instance, N. Eda et al., J. of Lightwave Technology, Vol. LT-3(2),pp. 400-407 (1985) show that the optimal value of the coupling parameterKL for a AR/AR phase shifted DFB laser is about 2, and suggest use of alower value of K in the front of the laser to improve the front facetoutput efficiency (K is the grating coupling constant and L is thelength of the laser "cavity", i.e., the distance between the front andback facets. By "AR/AR" is meant that the laser has both facetsanti-reflection coated).

H. Wu et al., Applied Physics Letters, Vol. 52(14), pp. 1119-1121(1988), discloses that in AR/AR DFB lasers relatively large values of KLlead to line broadening, and also discloses that a HR/AR laser has amore uniform field distribution than a AR/AR laser, resulting in reducedlongitudinal spatial hole burning. ("HR" designates a facet with highreflection coating). On the other hand, L. D. Westbrook et al., IEEE J.of Quantum Electronics, Vol. QE-21(6), pp. 512-518 (1985), discuss theexperimental determination of KL and report observation of narrowlinewidths even for relatively large (e.g., 2, 4 and 4.8) values of KL.

C. H. Henry, IEEE J. of Quantum Electronics, Vol. QE-21(12), pp.1913-1918 (1985) shows that in HR/AR DFB laser KL of about 1 is optimal,and KL≲2 is needed in order to attain high mode selectivity and quantumefficiency, and good insensitivity to reflections.

H. Soda et al., IEEE J. of Quantum Electronics, Vol QE-23(6), pp.804-814 (1987) give experimental results for AR/AR phase-shifted DFBlasers, and disclose that moderate coupling (KL˜1.25) is optimum tomaintain high mode selectivity above threshold. The phase-shift wassymmetrically placed. Best yield of acceptable lasers was also obtainedfor KL of about 1.25.

K. Utaka et al., IEEE J. of Quantum Electronics, Vol, QE-22(17), pp.1042-1051 (1986), teaches that in AR/AR phase shifted DFB lasersplacement of the phase-shift toward the front facet of the cavityimproves efficiency of power extraction, and the optimum structure interms of wavelength selectivity is one having the phase-shift at thecenter of the cavity, with both end reflectivities being zero.

Y. Kotai et al., Electronics Letters, Vol. 22, pp. 462-463 (1986) teachthat the output power asymmetry can be enhanced by placing thephase-shift near the front of the laser. See also F. Favre, ElectronicsLetters, Vol. 22(21), pp. 1113-1114 (1986), and S. McCall et al., IEEEJ. of Quantium Electronics, Vol. QE-21(12), pp. 1899-1904 (1985).

THE INVENTION

In a broad aspect the invention is a multichannel analog optical fibercommunication system that comprises a semicondutor laser having noveldesign features. The system comprises transmitting means, receivingmeans, and a length of optical fiber, signal-transmissively connectingthe transmitting means and the receiving means. The transmitting meanscomprise a semiconductor laser according to the invention, havingfeatures that are discussed below.

The operation of the system causing a current of predetermined value toflow the laser, and varying the current in response to an externalsignal, such that L varies in response to the external signal. Ingeneral the current has a dc component corresponding to the operatingpoint of the diode on the L-I curve, and an ac component superimposed onthe dc component. The operating point is selected at or near the currentvalue (herein designated I_(op)) at which dL/dI is a maximum. Operatingthe laser at or near I_(op) can result in relatively low signaldistortion, in particular, relatively low second order distortion, andmakes it possible to meet the very stringent specifications that aredeemed necessary for successful operation of multichannel analog fiberCATV trunk transmission systems under current consideration.

In a further broad aspect the invention is a semiconductor laser havingnovel design features. These features can increase the likelihood that agiven laser will have characteristics such that it is acceptable for usein a multichannel analog fiber communication system. Of course, lasersaccording to the invention are expected to find advantageous use also inother applications that can benefit from low distortion, and all suchuses are contemplated.

More particularly, lasers according to the invention comprise asemiconductor body that forms a laser cavity of length L that is definedby a front facet and a back facet (the "front" of the cavity is the sidefrom which the useful radiation is emitted, and the "back" is theopposite side). Assoicated with each of the facets is a reflectivity,and associated with the cavity (and any given portion thereof is, duringoperation of the laser, a photon density and a gain (by "operation" ofthe laser we mean above-threshold operation). Lasers according to theinvention also comprise a grating or other means for providingdistributed feedback, and further comprise contact means that facilitateflowing an electrical current through the semiconductor body.

The lasers furthermore comprise means adapted for producing, duringoperation of the laser, a non-uniform photo density in the cavity, thedensity of photons being larger in the back portion of the cavity thatin the front portion (the "back portion" of the cavity herein is theportion that is adjacent to the back facet, and the "front portion" isthe remainder of the cavity. The length of the back portion typically isthe approximate range L/4 to 3L/4, exemplarily about L/2). Thenon-uniform photon density is such that, during operation of the laser,the gain in the back portion of the cavity is substantially independentof the current through the semiconductor body, whereas the gain in thefront portion of the cavity is a function of the current through thesemiconductor body. Such a non-uniform photon distribution substantiallyenhances the likelihood that a laser according to the invention has avalue of I_(op) -I_(th) greater than some pre-determined value, e.g.,greater than 10 mA. (I_(th) here is the laser threshold current, i.e.,the device current at which the transition to laser oscillation occurs).A relatively high value of the parameter I_(op) -I_(th), signifies thatthe laser is likely to have relatively high output power at I_(op), andtherefore is likely to be acceptable for use in a multichannel analogoptical fiber communication system, or other application that requires alaser that exhibits low distortion at a relatively high output level.

Those skilled in the art know that DFB lasers typically have alongitudinally varying photon distribution. See, for instance, M. Wu etal, (op. cit.). However, the exact location of the peak in thedistribution depends on the phase of the wave at the laser facets. Thisphase is substantially uncontrollable and therefore typically randomlydistributed, resulting in wide variation of the location of the peakbetween lasers, even nominally identical lasers from a given wafer. Wehave discovered that, by incorporating into the laser design means thattend to position the peak of the photon distribution in the back portionof the laser cavity, yield of acceptable lasers can be substantiallyimproved. It is to be emphasized, however, that incorporation of meansthat favor the desired photon distribution can not insure that a givenlaser will have the desired distribution, due to the randon phases ofthe wave at the facets.

A possible reason for the observed yield improvement is as follows. Ifthe photon density above threshold is high in the back portion of thecavity, then the gain in that portion of the cavity is "clamped" nearthe threshold gain. On the other hand, the photon density in the frontportion of the cavity being relatively low, the gain in the frontportion increases above its threshold value as the drive current isincreased above I_(th). This increasing gain is believed to effectivelyincrease the slope efficiency (the maximum value of dL/dI) of the laser.The value of I_(op) -I_(th) is directly related to the degree to whichthe front of the laser cavity is disconnected from the back, and thedegree to which the gain in the front part is "unclamped". Thus, byproviding means that favor gain clamping in the back part of the laserand unclamped gain in the front, the likelihood that a given laser has arelatively large value of I_(op) -I_(th) (and thus is a likely candidatefor use in a multichannel analog optical fiber communication system) issubstantially increased over prior art lasers.

The above theoretical discussion is included for tutorial purposes onlyand is not intended to limit the invention.

Departing from prior art practice which generally aimed at increasedlaser power asymmetry and high slope efficiency in lasers for high bitrate communication systems, we have discovered that the yield of lasersacceptable for use in multichannel analog communication systems can besubstantially increased if the lasers are designed to have relativelylow (less than 5 or even 4) asymmetry. "Laser power asymmetry" isdefined herein as the ratio of front facet power to back facet power,determined at a front facet power of 4 mW. As a consequence of the novelrequirement of relatively low asymmetry, in a collection of lasers ofnominally identical parameters, the lasers that are typically mostsuitable for use in the referred to analog communication systems haverelatively low slope efficiency. Those skilled in the art will recognizethat there are design features that affect slope efficiency butessentially do not effect asymmetry.

We have identified several means for producing the desired non-uniformphoton distribution, and still others may be discovered later. Thesemeans can be used singly or in various combinations, as may be requiredby a particular application. However, lasers according to the inventiontypically will be relatively strongly coupled AR/HR DFB lasers (i.e.,having an antireflection coated front facet and a high reflection coatedback facet, with KL greater than about 1.6, typically in the range1.6-2.5, with values of about 2 being currently preferred. It will berecognized that the prior art frequently aimed at values of KL≲1.5 indistributed feedback (DFB) lasers for communications use. See, forinstance, C. H. Henry, (op. cit.).

The means adapted for producing the desired photon distribution alsoinclude a grating feature (conventionally called a phase shift) locatedin the part of the grating that is associated with the back portion ofthe cavity. The feature typically is an appropriate gratingdiscontinuity. In prior art lasers phase shifts typically were placed atthe mid-point of the grating, or in the front portion of the cavity.This typically was done in combination with AR/AR coatings to maximizethe spectral purity. The prior art phase shifts do not favor high photondensity in the back portion of the cavity.

Other means that favor the desired non-uniform photon distributioninclude split contact means, with one part of the split means beingassociated with the back portion of the cavity and another part beingassociated with the front portion of the cavity, such that the currentsthrough the back and front portions of the cavity can be separatelycontrolled. Still other means are structure means that have the effectof causing the gain in the front portion of the cavity to be relativelylow, as compared to the gain in the back portion. Exemplary of suchstructure means is a cavity of non-constant width, with at least a partof the front portion of the cavity being wider than the back portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary system according to theinvention, namely, a multi-channel sub-carrier multiplexed amplitudemodulated optical communication system, comprising a laser according tothe invention;

FIG. 2 shows an exemplary curve of light output intensity L versus laserinjection current I, and also shows L', the first derivative of L withrespect to I, all for a laser that has high asymmetry and relativelysmall value of I_(op) -I_(th).

FIG. 3 shows the same parameters as shown in FIG. 2, but for a laseraccording to the invention that has low asymmetry and a relative largevalue of I_(op) -I_(th), and thus is a candidate for use in theinventive system;

FIG. 4 schematically depicts the photon distribution in an exemplarylaser according to the invention;

FIGS. 5-7 schematically show exemplary lasers according to theinvention.

No attempt has been made to show true dimensions and/or proportions.Analogous features in different figures are designated by the samereference numerals.

DETAILED DESCRIPTION OF SOME CURRENTLY PREFERRED EMBODIMENTS

FIG. 1 schematically depicts an exemplary amplitude modulated-vestigialsideband signal sub-channel multiplexed optical transmission system 10.Several baseband frequency modulation television channel signals 120,121, 122, . . . 12n are frequency division multiplexed on differentcarrier frequencies ω₀, ω₁, ω₂ . . . ω_(n), (with n typicallysubstantially greater than 10, e.g., about 40) as separate amplitudemodulated-vestigial sideband signal sub-channels in a compositemultiplex signal. A summer 15 combines the individual channel signals atthe different sub-channel frequencies and a dc bias current I_(op) fromdc source 11 into the composite multiplex signal. This compositemultiplex signal is applied to inventive laser diode 18 as the laserdrive input signal.

The total laser input drive signal, or injection current, to the laserdiode 18 includes both the dc bias component I_(op) and the compositemultiplex signal from the summer 15. The channels typically are equallyspaced in frequency, with the frequency width of each channel typicallybeing in the range from 10-550 MHz, the bandwidth typically depending,inter alia, on the nature of the signal that is to be transmitted. Theoutput of the laser generally is in the visible or near infrared portionof the spectrum, exemplarily in the approximate range 0.8-1.6 μm.Currently preferred are wavelengths of about 1.3 and 1.55 μm,corresponding to the transmission "windows" of currently available SiO₂-based optical fibers. The output radiation is coupled into opticalfiber 13, and transmitted therethrough to receiver 14.

DFB lasers are well known in the art and need no detailed discussion.See, for instance, J. L. Zilko et al., IEEE J. of Quantum Electronics,Vol. 25 (10), pp. 2091-2095 (1989), incorporated herein by reference.Methods of producing a phase-shift (typically a λ/4 shift) are alsoknown. See, for instance, K. Utaka, (op. cit.), incorporated herein byreference. Furthermore, those skilled in the art know techniques forapplying AR coatings (typically reflectivity less than about 2%) and HRcoatings (typically reflectivity greater than about 30%) to laserfacets. Herein we consider a laser to be AR/HR if the ratio of frontfacet reflectivity to back facet reflectivity is less than about 5%.

FIG. 2 shows data obtained from a laser that is not consideredacceptable for use in a multichannel analog communication system, butthat is potentially useful for a digital communication system. Inparticular, curve 20 is the front facet output power, curve 21 is theback facet output power, and curve 22 is the first derivative of 20 withrespect to the drive current. The laser had a relatively high value oflaser power asymmetry (7.8), relatively low value of I_(op) -I_(th) (8mA), and relatively high slope efficiency (0.32 mW/mA).

FIG. 3 shows data from a laser according to the invention that isacceptable for use in a multichannel analog communication system. Curves30, 31, and 32 are the front facet output power, back facet outputpower, and the first derivative of the front facet output power,respectively. The laser has relatively low power asymmetry (2.3), andrelatively high value of I_(op) -I_(th) (17 mA). It also has relativelylow slope efficiency (0.24 mW/mA).

The laser of FIG. 3 is a CMBH-DFB laser of the type described by J. L.Zilko et al., (op cit.). The laser had a 65% HR coating and a (<1%) ARcoating and Kl of about 1.8.

Lasers according to the invention optionally comprise one or moreadditional features that can enhance the likelihood that a given laserhas the desired non-uniform photon distribution and thus is likely to beacceptable for use in an analog communication system. An exemplarynon-uniform photon distribution according to the invention isschematically depicted in FIG. 4.

One feature adapted to enhance the likelihood of obtaining the desirednon-uniform photon distribution is a phase shift (typically by λ/4) inthe back portion of the grating, i.e., in the part of the grating thatis associated with the back portion of the cavity. A phase shift allowsthe lasing mode to be concentrated about the phase shift. Locating thephase shift in the back portion of the grating can greatly increase theprobability that the laser has a "sweet spot" (and has I_(op) muchgreater than I_(th)), and is contrary to prior art practice, whichplaces the phase shift into the center or the front portion of thegrating. FIG. 5 schematically depicts a AR/HR laser (50) with a phaseshift located approximately 3L/4 from the front facet. The semiconductorbody comprises active region 52 and grating 53, with the lattercomprising λ/4 phase shift 54. The front facet 55 is (completely orpartially) covered with AR coating 57 and back facet 56 is (completelyor partially) coated with HR coating 58. The laser also comprisesconventional contacts 59 and 59' .

Another such feature is separate contacts for the front portion and theback portion of the laser, such that the current density through thefront portion is separately controllable from that through the backportion. FIG. 6 schematically shows laser 60 which comprises splitcontact means 61 and 62. It will be understood that such a laser mayoptionally comprise facet coatings and/or a phase shift (not shown).

By appropriately dividing the drive current between the front and backportions of the laser, it is possible to insure that the back portionreaches threshold first. Assuming that the facet phases of a given laserare appropriate, such current control can be used to enforce a "sweetspot" or to increase the value of I_(op) -I_(th). As those skilled inthe art will recognize, a current divider can be integral to the laser,or the division can be caused by external means. Also, they willrecognize that the division can be in a fixed ratio or be some morecomplicated function. As the limiting case of the divided current, alaser can have an unpumped front portion.

A still further feature is structural means that cause the gain in thefront portion of the laser to be relatively low, as compared to the gainin the back portion. Exemplary structural means are a non-constant mesawidth, with the front portion of the mesa being wider than the backportion. The change in width can be gradual or abrupt. FIG. 7schematically depicts an exemplary laser 70 with non-constant mesawidth. The semiconductor body comprises active region 52 and grating 53,both being part of mesa 72. The body also comprises semi-insulatingcurrent blocking material 73. The mesa is etched to comprise arelatively narrow portion adjacent to the rear facet and a relativelywide portion 74. Facet coatings are not shown.

EXAMPLE 1

A multiplicity of CMBH DFB lasers was produced substantially asdescribed by Zilko et al. (op. cit.). The grating depth and devicelength were chosen to result in KL of about 1.8. Yttria stabilizedzirconia coatings were applied to the facets, substantially as describedin U.S. Pat. No. 4,749,255 (which is incorporated herein by reference).The coating parameters were selected to result in an AR coating (<1%reflectivity) on the front facets and a HR coating (about 65%reflectivity) of the back facets. Conventional electrodes were appliedin a conventional manner, and a variety of measurements were carried outon the devices. The measurements indicate that the multiplicity oflasers according to the invention contained a higher percentage oflasers adapted for use in a multichannel analog optical fibercommunication system that is present in an analogous sample of prior artCMBH DFB AR/AR lasers that have KL<1.6. FIG. 3 shows measurement resultsfrom a representative member of the multiplicity of lasers. Themeasurements in their totality indicate that the lasers that met thespecifications for the communication system had a non-uniform photondensity, with the density being larger in the back portion of the cavitythan in the front portion.

EXAMPLE 2

A multiplicity of lasers is produced substantially as described inExample 1, except that a λ/4 phase shift is introduced into the gratingin a manner substantially as taught by Utaka et al. (op. cit.).Lithography conditions are chosen to result in a phase shift locationabout 3L/4 from the front facet. Measurements carried out on themultiplicity of lasers yield results similar to those carried out on thelasers of Example 1.

EXAMPLE 3

A multiplicity of lasers is produced substantially as described inExample 1, except that the p-side electrode is a symmetrically splitelectrode. The split electrode is produced by lithography and etchingtechniques that are well known in the art. Electrical isolation betweenthe front and back electrode segments of 2-5 kΩ was achieved by meanssubstantially as described by C. Y. Kuo et al., Applied Physics Letters,Vol. 55 (13), pp. 1279-1281 (1989). Injection of independent currentsthrough the two electrode segments of a given laser made it possible toclamp the gain in the back portion of the cavity while at the same timehaving unclamped gain in the front portion. Measurements carried out onthe multiplicity of lasers yield results similar to those carried out onthe lasers of Example 1.

EXAMPLE 4

A multiplicity of lasers is produced substantially as described inExample 1, except that a mask set is used that is appropriately modifiedto result in mesas of the type schematically shown in FIG. 7.Conventional lithography and etching are used to produce the mesas, withthe front portion of a given mesa being about double the width of theback portion. This geometrical feature results in lowered gain in thefront portion of the cavity, with the gain in that portion beingdependent on the laser current. Measurements carried out on themultiplicity of lasers yield results similar to those carried out on thelasers of Example 1.

We claim:
 1. A semiconductor laser comprising(a) a semiconductor bodyforming a radiation cavity of length L defined by a front facet and aback facet, associated with each of said facets being a reflectivity,and associated with the cavity during operation of the laser is a photondensity and a gain; (b) a diffraction grating; and (c) contact meansfacilitating flowing an electrical current through the semiconductorbody; characterized in that the laser further comprises (d) meansadapted for producing a non-uniform photon density in the cavity, withthe density of photons being larger in the portion of the cavity that isadjacent to the back facet of the cavity (to be referred to as the"back" portion) than it is in the remaining portion of the cavity (to bereferred to as the "front" portion), such that during operation of thelaser the gain in the back portion is substantially independent of thecurrent through the semiconductor body whereas the gain in the frontportion of the cavity is a function of the current through thesemiconductor body.
 2. The laser of claim 1, having a laser powerasymmetry less than about 5, where "laser power asymmetry" is defined asthe front facet output power (L) divided by the back facet output power,measured at 4 mW front facet output power.
 3. The laser of claim 1,comprising a high reflection coating on the back facet and anantireflection coating on the front facet, and furthermore comprising adiffraction grating selected such that KL is in the approximate range1.6-2.5, where K is the grating coupling constant.
 4. The laser of claim3, further comprising a diffraction grating discontinuity (termed a"phase shift"), the phase shift being located in the part of the gratingthat is associated with the back portion of the cavity.
 5. The laser ofclaim 3, wherein the current through the laser comprises a first currentthrough the back portion and a second current through the front portion,and wherein the contact means are split contact means that comprisefirst and second contact means associated with the back portion and thefront portion of the cavity, respectively.
 6. The laser of claim 1,wherein the cavity has non-constant width, with at least a part of thefront portion of the cavity being wider than the back portion. 7.Multichannel analog optical fiber communication system comprisingtransmitting means, receiving means, and a length of optical fibersignal-transmissively connecting the transmitting means and thereceiving means, the transmitting means comprising at least one thatcomprises(a) a semiconductor body forming a radiation cavity of length Ldefined by a front facet and a back facet, associated with each of saidfacets being a reflectivity, and associated with the cavity duringoperation of the laser is a photon density and a gain; (b) a diffractiongrating; and (c) contact means facilitating flowing an electricalcurrent through the semiconductor body; characterized in that the laserfurther comprises (d) means adapted for producing a non-uniform photondensity in the cavity, with the density of photons being larger in theportion of the cavity that is adjacent to the back facet of the cavity(to be referred to as the "front" portion), such that during operationof the laser the gain in the back portion is substantially independentof the current through the semiconductor body whereas the gain in thefront portion of the cavity is a function of the current through thesemiconductor body.
 8. The communication system of claim 7, wherein thelaser has a laser power asymmetry less than about 5 and a relatively lowslope efficiency, where "laser power asymmetry" is defined as the frontfacet output power (L) divided by the back facet output power, measuredat 4 mW front facet output power, and where "slope efficiency" is themaximum value of dL/dI, with I being the laser current.
 9. Thecommunication system of claim laser of claim 7, wherein the lasercomprises a high reflection coating on the back facet and anantireflection coating on the front facet, and furthermore comprises adiffraction grating selected such that KL is in the approximate range1.6-2.5, where K is the grating coupling constant.
 10. The communicationsystem of claim 9, wherein the laser further comprises a diffractiongrating discontinuity (termed a "phase shift"), the phase shift beinglocated in the part of the grating that is associated with the backportion of the cavity.
 11. The communication system of claim 9, whereinthe current through the laser comprises a first current through the backportion and a second current through the front portion, and wherein thecontact means are split contact means that comprise first and secondcontact means associated with the back portion and the front portion ofthe cavity, respectively.
 12. The communication system of claim 7,wherein the laser cavity has non-constant width, with at least a part ofthe front portion of the cavity being wider than the back portion.